Projection systems used for projecting an image on a screen can use multiple color light sources, such as light emitting diodes (LED's), with different colors to generate the illumination light. Several optical elements are disposed between the LED's and the image display unit to combine and transfer the light from the LED's to the image display unit. The image display unit can use various methods to impose an image on the light. For example, the image display unit may use polarization, as with transmissive or reflective liquid crystal displays.
Still other projection systems used for projecting an image on a screen can use white light configured to imagewise reflect from a digital micro-mirror (DMM) array, such as the array used in Texas Instruments' Digital Light Processor (DLP®) displays. In the DLP® display, individual mirrors within the digital micro-mirror array represent individual pixels of the projected image. A display pixel is illuminated when the corresponding mirror is tilted so that incident light is directed into the projected optical path. A rotating color wheel placed within the optical path is timed to the reflection of light from the digital micro-mirror array, so that the reflected white light is filtered to project the color corresponding to the pixel. The digital micro-mirror array is then switched to the next desired pixel color, and the process is continued at such a rapid rate that the entire projected display appears to be continuously illuminated. The digital micro-mirror projection system requires fewer pixelated array components, which can result in a smaller size projector.
Image brightness is an important parameter of a projection system. The brightness of color light sources and the efficiencies of collecting, combining, homogenizing and delivering the light to the image display unit all affect brightness. As the size of modern projector systems decreases, there is a need to maintain an adequate level of output brightness while at the same time keeping heat produced by the color light sources at a low level that can be dissipated in a small projector system. There is a need for a light combining system that combines multiple color lights with increased efficiency to provide a light output with an adequate level of brightness without excessive power consumption by light sources.
Liquid crystal displays often use only one polarization of light, and generally use light sources that generate unpolarized light. Several means for converting unpolarized light to polarized light, including polarizing beam splitters, and arrays of birefringent or polarizing prisms have been used to generate polarized light. These systems can require a substantial volume, especially considering light collection optics and integrating systems. Several types of displays, including handheld devices, micro-projection systems, and head-mounted displays require both compactness and high efficiency.
Pico and pocket projectors have limited available space for efficient color combiners, light integrators, and/or homogenizers. As a result, efficient and uniform light output from the optical devices used in these projectors (such as color combiners and polarization converters) can require compact and efficient optical designs.